Compression of frequency chirped ultrashort laser pulses can create pulses of high peak power and short pulse duration. In particular, pulses down to the record value of 6 femtoseconds (fs) can be obtained from 50 fs pulses out of a mode-locked dye laser by self-phase modulation in a single-mode optical fiber and subsequent chirp compensation using suitable phase dispersive elements. The use of single-mode fibers is, however, limited to low energy (nanoJoule) pulses due to both damage of the material and the appearance of higher-order nonlinearities that introduce distortions on the pulse which cannot be easily compensated.
The recent availability of femtosecond pulses of high energy, microJoules, milliloules and even Joules, from solid state lasers systems calls for a new pulse spectral broadening technique. So far, spectral broadening of high energy femtosecond pulses has only been accomplished by unguided propagation in bulk materials. Except for low energy pulses, unguided propagation in bulk materials leads to filamentation of the spatial mode and thus poor spatial mode quality. Additionally, the nonuniform transverse intensity profile of the beam leads to nonuniform transverse self-phase modulation. As a consequence, effective pulse compression of these pulses can only be achieved using the central and more uniform fraction of the beam profile.
There is a commercial need for sources of high peak power laser pulses that have a pulse duration shorter than the duration of the effect being studied, and/or shorter than the process being induced by the interaction of these short laser pulses with matter. It is especially important in the field of chemistry, for example, to generate pulses of extremely short duration at specific wavelengths that correspond to the electronic vibrational state of the sample being studied. The present modality for producing these extremely short pulses is to start by creating pulses of approximately the desired duration in a Ti:Sapphire laser oscillator, and amplify them in a multipass amplifier, regenerative amplifier, or chirped pulse amplifier. Products for generating such pulses are commercially available. See for example, 20 femtoseconds pulse width produced in a modified version of the Clark-MXR, Inc., Model NJA-5 Ti:Sapphire Laser Oscillator, the Clark-MXR, Inc., Model CPA-1000 Ti:Sapphire Regenerative Amplifier incorporating a spectral filter as described in U.S. Pat. No. 5,592,327 to suppress gain. The complexity of this approach combined with the increased number of components that depend on nonlinear processes as well as their sensitivity to environmental changes makes these systems extremely sensitive to even small environmental changes.
Our invention offers a novel technique for generating the additional bandwidth needed to create pulses of high energy with good spatial mode quality. It uses a guiding element for confining the propagation of an input pulse to a large single mode diameter and thus maintains a high quality spatial mode profile on the beam as it propagates over long interaction lengths. It uses a fast non-linear medium for generation of the additional spectral bandwidth needed to create a pulse whose width is shorter than that of the input pulse, yet at the same time has a high threshold for multiphoton ionization and consequently low absorption. It can be employed as an add-on to existing ultrashort, high energy sources operating in a pulse width regime of substantially longer duration, and is thus less sensitive to environmental perturbations. Lastly, it is scaleable to extremely high peak powers--which opens up the possibility of performing new experiments in high-field light/matter interactions.